Skip to main content
Chemistry LibreTexts

Experiment

Equipment

For a real cell where the time constant of the cell is fairly long (i.e., several milliseconds) an analog potentiostat with an inexpensive digital oscilloscope for data acquisition with a printer readout is recommended. With a computer-controlled potentiostat, fast data acquisition with readout is part of the instrument.

  • An analog or computerized potentiostat with appropriate data acquisition capability is needed. Please see your laboratory instructor about the potentiostat and accompanying manual.
  • Test circuit
    • 100 Ω and 10,000 Ω resistors
    • 2 μF capacitors (not electrolytes)
  • Small volume (< 5 ml) cell and electrodes
    • 3 – 4 mm diameter glassy carbon or Pt planar tip working electrode
    • Ag/AgCl reference electrode
    • Pt auxiliary electrode
    • Small volume cell
    • Electrode polishing kit
  • Pipettes and other laboratory glassware and supplies

Chemical Solutions:

  • 100 ml of 5 mM potassium ferricyanide in 0.1 M KNO3
  • 100 ml of 0.1 M KNO3 solution
  • 10 ml of 5 mM ferrocene carboxylic acid (FCA) in pH 7 buffer (provided by instructor)
  • Pure water for dilutions
    Note: The concentrations of ferricyanide and FCA should be known to 3 significant figures.

Procedure

  1. Test of RC circuit –
    1. Connect in series a 100 Ω resistor, a 10,000 Ω resistor and a 2 μF capacitor. Use a solid-state capacitor, not an electrolytic one.
    2. The AE connector from the potentiostat is attached to the 100 Ω resistor, the RE between the two resistors, and WE to the end of the capacitor:

      \[\begin{align}
      \ce{AE > — [R,\: 100\, Ω] &\textrm{——} [R,\: 10\, KΩ] \textrm{——} [C,\: 2\, μF]→WE} \tag{5}\\
      &\:\:↑\\
      &\:\:\ce{RE}
      \end{align}\]

      The 100 Ω resistor helps to stabilize the potentiostat at short rise times (< 20 μS).

    3. Please obtain directions for the operation of the potentiostat from the instructor.
    4. Set the potential and current measurement parameters on the potentiostat to the following values:

      Ei = 0.00 V            Current scale: 50 μA

      E1 = 0.500 V         Filter: < 100 μS

      E2 = 0.00 V

      Operate the potentiostat in the chronoamperometry mode

      If the time duration of E1 can be set on the potentiostat, a 100 – 1,000 mS time duration is sufficient, depending on the RC time constant. Exact timing is not critical so that you can start/stop the experiment within 1 or 2 seconds.

      The instantaneous current is given by E1/R = 50 μA. The current decreases exponentially.

  2. Determine the value of the diffusion coefficient, D
    1. With a 3 mm diameter disk electrode (either GC or Pt), fill the cell with a 0.1 M KNO3 solution.
    2. Step the potential from 0.0 V to +0.500 V for 200 mS and then step back to 0.0 V. Repeat the experiment and record the i-t profiles.
    3. Repeat with a 5 mM ferricyanide in 0.1 M KNO3 solution. Step the potential from an initial 600 mV to 0.0 V for 300 mS and then return the potential to 600 mV. Wait 5 minutes and then repeat the experiment. Save and record the i-t profiles.

      Repeat the procedure with a 5 mM ferrocene carboxylic acid solution in a pH 7 buffer solution, stepping the potential from 0.0 V to 600 mV for 300 mS and then back to 0.0 V. Wait 5 minutes and then repeat the experiment. Save and record the i-t profiles.

      Please note that the potential is stepped from 600 mV to 0 V for ferricyanide, a reduction, whereas the potential is stepped from 0 V to 600 mV for ferrocene carboxylic acid, an oxidation.